Rod guides for centralizing sucker rods within production tubing are known in the prior art. A pumping unit has attached thereto a sucker rod and the sucker rod is coupled at its bottom end to a reciprocating pump. As the pumping unit moves the sucker rod down, the barrel of the reciprocating pump fills with the production fluid to be produced. Conversely, as the pumping unit moves the sucker rod up, a valve in the reciprocating pump shuts and the production fluid in the pump barrel is lifted, displacing production fluid above it and forcing one pump-barrel's worth of production fluid out of the hole.
The sucker rod must extend from the pumping unit all the way down to the reciprocating pump, which may be several thousand feet below the surface. Consequently, the sucker rod is subjected to a variety of stresses: compression, tension, torsion, and bending. During reciprocation, the string of the sucker rod tends to contact the walls of the pipe which surrounds it, resulting in abrasion of the sucker rod and surrounding tubing. This is particularly prevalent in deviated holes where, without the use of rod guides, the sucker rod would continuously contact and abrade against the tubing.
In operation, the sucker rod is immersed in production fluid. As the sucker rod moves up and down to pump fluid from down hole, the rod guide develops resistance to the movement of the sucker rod due to hydraulic action of the fluid around the rod guide.
Some rod guides are molded in place on the sucker rod. The rod is laid across a mold half, the other mold half is placed on top of the rod, and the combination is injected with a polymeric fluid that solidifies into a molded rod guide. Other rod guides are made as a separate unit apart from and installed on the sucker rod, particularly in the field to replace worn guides. Such rod guides are often termed "field-installable" rod guides. These guides may be either injection molded, machined from a stock material, or extruded and machined to achieve the desired structure.
One prior art field-installable rod guide is made from an extruded, solid cylindrical stock. The stock comes in the form of a long, solid bar of long-chain polymeric material. The bar stock is cut to the length of the desired rod guide, a cylindrical hole is cut in the center of the cut bar stock to a radius slightly less than the rod to which the guide will be attached, an axial access channel is cut to provide access of the rod to the center-hole, and the ends of the guide are beveled to reduce fluid friction. The guide is then ready to be snapped or hammered onto a rod, either at the yard or in the field.
This known field-installable rod guide is simple to make and is relatively inexpensive. Since the guide is made of extruded material, the intended applications of such a rod guide are relatively low temperature and low stress. Unfortunately, polymeric compositions that are easily extruded are not generally well suited for high temperature, high pressure, and high stress applications. Further, these guides suffer from several additional drawbacks. The solid-cylindrical aspect of such a rod guide delivers unnecessarily high fluid resistance to pumping movement in both the up and down directions, despite the beveled ends of the guide. Also, the round hole in the center of the guide does not adequately grip the rod and, consequently, the guide tends to slip on the rod. If a guide slips enough, a number of guides may become bunched at one end of a rod segment. This means that a long segment of the rod has no rod guide along its length and the rod may ride against the casing, particularly where the hole is not straight. This defeats the purpose of the rod guide.
Another drawback of this known rod guide is in the structure of the axial access channel that is cut in the rod guide to allow the guide to be placed upon the rod. As the guide is placed on the rod, the sides of the channel are forced apart enough for the guide to slip on the rod. As the sides of the channel are stretched apart, the portion of the guide directly opposite the channel experiences the stretch and this portion of the guide may exceed the elastic limit of the material. If this happens, the guide remains stretched and loses much of its ability to grasp the rod. One proposed solution to this problem has been to cut a second channel, wider yet shallower than the first channel, so that only part of the channel depth offers the narrower opening. The deeper part of the channel offers more material around the rod but this proposed solution has proven only partly satisfactory since the narrow channel still stretches the rod material, often beyond its elastic limit.
Thus, there remains a need for a field-installable rod guide that is simple and inexpensive yet firmly grasps the rod to remain fixed in place during normal operations. Such a rod guide should permit more fluid to pass adjacent the rod guide during reciprocating movement to reduce fluid drag against the rod guide. The guide should also provide a structure that recognizes the problem of exceeding the elastic limit of the guide material during installation and make allowances for this phenomenon.